The second path to search for life in the Universe is to look
for traces of the natural products of this life – the so-called
biosignatures [1]}, [2]}.
An excellent introductory reading material can be found at the
NASA Astrobiology
pagehttps://astrobiology.nasa.gov/education/alp/what-is-a-biosignature/.
The Oxygen and Ozone are the most informative biosignatures.
Strictly speaking, water is not a biosignature, but it is an
important solvent and its presence is considered critical for
life as we know it. The Carbon dioxide and Methane are other
potential indicators of life, although some geological sources
can contribute to their content in the atmospheres of exoplanets
[3]}, [4]}.
